Microelectronic packaging of electronic devices typically includes a die, a mounting substrate, electrical interconnections and an encapsulant to protect the electrical interconnections. Electronic devices having special requirements, such as alignment, heat dissipation, fluidic connection, impact shielding, and etc. can also impose corresponding additional requirements for the microelectronic packaging of the device. In some cases, such requirements can be solved using a mounting substrate formed of dissimilar materials. For example, US Patent Application Publication 2008/0149024, incorporated by reference herein in its entirety, describes a mounting substrate for an inkjet printhead die where a mounting assembly is made by insert molding. The mounting assembly includes a die mounting substrate for the mounting of the printhead die, a support region that provides alignment features, and a support for a flex circuit. In the insert molding process, the die mounting substrate (formed of ceramic, for example) can be placed in an injection molding tool and then molded plastic is formed around the die mounting substrate. Such a printhead die mounting assembly is an example of a die mounting assembly formed of dissimilar materials.
While the examples described herein will relate to inkjet printheads, it is contemplated that die mounting assemblies formed of dissimilar materials are not restricted to inkjet printheads. In particular, it is contemplated that electrical interconnections can be located in a region of a die mounting assembly near the boundary between two materials having different thermal expansion coefficients. As a result, the encapsulant that is deposited over the electrical interconnections can inadvertently bridge across the boundary between the two materials. Subsequent heating and/or cooling cycles (including cooling after the curing of the encapsulant at elevated temperature) can cause the encapsulant to crack. Such cracks can compromise the environmental protection provided by the encapsulant and therefore impair the reliability of the assembled device.
What is needed is a die mounting assembly that improves the reliability of the assembled device by keeping encapsulant from inadvertently bridging between two regions having different coefficients of thermal expansion.